This invention relates to recovering precious metal values from refractory ores, which include carbon- and sulfur-containing components, and to the control of environmental emissions during the treatment of those ores. In particular, this invention relates to a method of roasting those ores.
The purpose of roasting precious metal ores, such as gold ore, is to release for extraction the small particles of precious metal that are surrounded by refractory stone or minerals. Refractory refers to non-conventional ores, such as oxide, which implies extreme process measures must be taken to extract the metal. The roasting simply opens up passages for the penetration of a leaching solution into the interior of the ore particles. This is accomplished by the removal by volatilization or formation of volatile oxides of certain constitutents such as sulfur, arsenic or antimony.
For example, the gold in refractory sulfide ores is angstrom-sized and physically locked in the arsenian pyrite mineral species. Roasting of this ore oxidizes the sulfide mineral and changes the structure, which allows the cyanide leaching solution to come into contact with the gold. Temperature is an important parameter. High temperatures tend to form a dense particle rather than a "spongy" calcine. The dense particles trap the smaller precious metal particles, and result in lower metal recoveries. High temperature can cause melting of some components, which also results in metal encapsulation.
In the case of recovering gold from gold ores, roasting of refractory gold ore concentrates has been practiced for decades. Multiple hearth, rotary kiln and muffle reactors were first used for roasting. Fluid bed roasting provided a low-capital cost, low-maintenance technology with better process control and soon became the favored technology. The first fluid bed concentrate roasters were commissioned in the late 1940's. Early fluid beds were "bubbling" type. Environmental considerations did not significantly impact on the design. Feedstocks were highly exothermic and reaction rates were relatively rapid.
Roasting today must compete with other technologies for treatment of refractory ores. Ore bodies which are not amenable to concentration must be handled. Foremost, processing must be done in an environmentally acceptable manner.
Table 1 presents some of the minerals commonly present in refractory gold ores. Many of these minerals include sulfur and other elements that may require costly processing and disposal. In addition, ores may contain organic carbon. This carbon may have "preg robbing" characteristics, which takes up or "robs" the solubilized gold from being recovered during gold leaching operations.
TABLE 1 MINERAL ASSOCIATED WITH GOLD ORES NON-SULFIDIC SULFIDIC Name Formula Name Formula Quartz SiO.sub.2 Pyrite FeS.sub.2 Dolomite CaCO.sub.3.MgCO.sub.3 Pyrrhotite Fe.sub.5 S.sub.6 to Fe.sub.16 S.sub.17 Calcite CaCO.sub.3 Arsenopyrite FeAsS Muscovite K.sub.2 O.2Al.sub.2 O.sub.3.6SiO.sub.2.2H.sub.2 O Orpiment As.sub.2 S.sub.3 Albite Na.sub.2 O.Al.sub.2 0.sub.3.6SiO.sub.2 Realgar AsS Talc 3MgO.4SiO.sub.2.H.sub.2 O Tetrahedrite 4Cu.sub.2 S.Sb.sub.2 S.sub.3 Clay Al.sub.2 O.sub.3.(x)SiO.sub.2.(y)H.sub.2 O Chalcopyrite CuFeS.sub.2 Calaverite AuTe.sub.2 Sphalerite ZnS Petzite Ag.sub.3 AuTe.sub.2 Galena PbS Gold Au Stibnite Sb.sub.2 S.sub.3 Scorodite FeAsO.sub.4.2H.sub.2 O Enargite Cu.sub.3 AsS.sub.4 Selenium Se Cinnabar HgS
Environmental issues which must be addressed are primarily the fate of the sulfur gases, arsenic, and mercury. Other pollutants such as antimony may be important depending on the specific ore mineralogy.
High concentrations of sulfur gases, primarily sulfur dioxide, will be present in the exhaust gases from all concentrate roasters. Generally, the concentration of these sulfur oxide gases should be substantially reduced prior to discharge to atmosphere. One option is the manufacture of sulfuric acid. A second option would be a wet scrubbing system using alkali. Because of the low value of sulfuric acid, very few plants utilize the first option; however, that decision also depends on the availability of a market for the sulfuric acid, and the cost to dispose of the sulfur otherwise.
In the case of concentrates with high arsenic contents efforts have been made to volatize the arsenic as arsenic trioxide. This results in higher gold recoveries. There are several technologies available for the removal of arsenic trioxide from the exhaust gases.
The roasting of whole or unconcentrated ores has also been commercialized. There are several characteristics of the whole ores that differ from concentrates, which significantly affect design. The ore has a low heating value. Dry feeding of the ore is required, whereas most concentrates are fed in a slurry form. Reaction rates are slower with whole ores, thus requiring long solids retention time. Whole ore, as opposed to concentrates, can have a higher variability in the amount of sulfur, and therefore requires blending of different ore lots to the roaster feed. But, blending ores to obtain consistent overall sulfur content can be problematic, and therefore, alternative methods may be required to help control SO.sub.2 content.
Because the sulfur gases may cause some environmental problems, there must be additional processing steps taken with whole ore roasting to meet regulatory compliance. One solution is to scrub the roaster off gases with an alkali. But, whole ore roasting produces more dilute SO.sub.2 gases, and dilute gases are difficult to scrub and remove. Another solution that has been practiced is to add lime in the roaster to capture the sulfur "in situ," i.e. by forming solid sulfates. Yet another solution suggested has been to add soda ash (sodium carbonate) in the roaster to control the SO.sub.2 emissions. In some applications, however, soda ash may cause other problems such as generation of fines, due to its friability.
With low gold prices, the cost of those chemicals becomes more expensive relative to the value of the gold being recovered from the ore. Thus, there is a need for other solutions to the environmental issues that are more cost effective, and offer potential benefits of enhancing the recovery of the precious metals.